Stephen Harper is stepping up his attack on rival Justin Trudeau’s musings about the “root causes” behind the Boston bombings, saying the only appropriate reaction to such attacks is to condemn the actions and direct government efforts to fighting them.

“This is not a time to commit sociology, if I can use an expression,” Mr. Harper told a news conference in Ottawa, a phrase he later repeated in French for the benefit of French-language media outlets.

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Mr. Harper had been asked by a journalist Thursday to say at what point he considered it acceptable to start talking about the “root causes” that might lead someone to plot an attack on North American soil, such as the Canadian residents arrested this week and accused of scheming to derail a Via train.

The Prime Minister made the remark on the same day a fierce debate erupted over news that Tory MPs are being urged to blanket their ridings with flyers bashing Mr. Trudeau as an inexperienced lightweight.

“Root causes” is the phrase Mr. Trudeau used last week when he said it was essential to look at the motivating factors behind the Boston Marathon bombings. Mr. Harper wasted little time in ridiculing his Liberal opponent for what he considered a weak response to terrorism.

On Thursday, the Prime Minister elaborated on his assertion that now is no time for academic pondering, saying that those who would seek to hurt Canada are starkly opposed to Western values.

“These things are serious threats – global terrorist attacks, people who have agendas of violence that are deep and abiding threats to all the values that our society stands for,” the Prime Minister said.

“I don’t think we want to convey any view to the Canadian public other than our utter condemnation of this violence and our utter determination through our laws and through our laws and activities to do everything we can to counter it.”

To devote this much time to the leader of the third-biggest party in the Commons suggests Mr. Harper is stooping to conquer – but Conservatives say privately they believe the opportunity to brand Mr. Trudeau as inexperienced is too good to pass up.

The Tories have recently begun running attack ads that brand the Liberal Leader as inexperienced or “in over his head” and the Conservatives feel Mr. Trudeau has confirmed this criticism.

Mr. Harper defended the use of taxpayers’ dollars to finance a bulk-mail campaign – known as 10-per-centers – against Mr. Trudeau at a news conference on Thursday. He said the campaign is well within the rules of the House of Commons, and MPs from all parties send partisan missives.

“All parties work within those rules, and all parties use those activities and use those rules.”

But the newly minted Liberal Leader is hitting back, accusing the Tories of using the public purse to spread distortions and lies.

“Instead of defending an increasingly indefensible, mediocre record on the economy and on various decisions, they attack and they use whatever public resources they can to turn people away from politics and to foster cynicism,” Mr. Trudeau said in Happy Valley-Goose Bay.

Separately, Thursday, the Combatting Terrorism Act, a bill that would give additional police powers at the cost of civil liberties, received Royal Assent. The Harper government, which sponsored the legislation, did not say how soon S-7 comes into force.